Equalization of an audio signal provided to a loudspeaker may be used to control a frequency response. For example, when a loudspeaker tends to output sound at a first frequency range at a lower amplitude relative to other frequencies, the first frequency range may be amplified in the input signal, to compensate for the performance of the loudspeaker and to yield a flatter frequency response.
In addition to, or alternatively to, flattening a frequency response, equalization may be used to modify the frequency response to conform to aesthetic preferences and/or characteristics of an environment where the loudspeaker is located. For example, some types of music may be played with lower frequencies enhanced in case the music comprises more information at said lower frequencies.
When listening to sound, such as music, output from at least one loudspeaker in a room, characteristics of the room may affect the listening experience. Such characteristics include physical dimensions of the room, materials used in the walls, roof and floor, and items such as furniture disposed in the room. Also placement of loudspeakers and the listener relative to the room and each other may affect the listening experience. The listening experience may be perceptibly different at different locations in the room.
When a room is capable of reflecting sound, using a loudspeaker in the room may give rise to resonances, also called room modes, or modes, in the space enclosed by the walls and/or the roof and floor. A mode occurs at a frequency corresponding to a multiple of half a wavelength, whereby the dimensions of the room define the frequencies that may resonate in that room. The smaller the room, the higher the lowest mode frequency. As a consequence, larger rooms may have lowest resonances that are too low for humans to hear. Also, as modes occur at frequencies related to the room dimensions, there are also modes at higher frequencies. The density of room modes, defined as the number of room modes per unit of frequency, tends to increase as the frequency increases.
A room mode may store energy. Energy of the sound output by the loudspeaker is stored in the vibration sustained by the mode. In case the loudspeaker is switched off, the room mode will attenuate at a rate determined by, among other things, absorbance of the room. As room modes may affect the listening experience, controlling the energy stored in modes is of use. Control methods include design of the room geometry, covering the walls with materials having absorbance at the frequencies where mode resonances exist, and equalizing loudspeaker output to be low at the mode frequencies, so that the sound level at the room modes will not grow larger than the sound level in the frequencies close to the mode, hence producing equalization of the magnitude change effected by the mode.
Document WO99/66492 discloses a sound reproduction apparatus for reducing the level of acoustical reflections in a room. Document EP1322037 discloses a method for designing a modal equalizer for a low frequency sound reproduction. Document EP1516511 discloses a method for designing a modal equalizer for a low frequency audible range especially for closely positioned modes.